1. Field of the Invention
This invention relates to an optical device for limiting transmitted power by means of induced absorption. Cr-doped crystalline solids and solutions of organic compounds suitable for such a device exhibit an absorption coefficient that increases with increasing intensity.
2. Description of the Prior Art
For some applications of lasers, it is desirable to have uniform output power over time. Both passive and active techniques for minimizing output fluctuations have been disclosed. A passive technique based on the Rayleigh effect in benzene was disclosed by R. H. Pantell et al., Appl. Phys. Lett. 11, 213 (1967).
Active techniques, which involve sampling the laser output and using the detected signal to control the voltage on a Pockels cell in the laser cavity, have been disclosed by F. R. Marshall et al., Proc. IRE 50, 2108 (1961); H. Statz et al., J. Appl. Phys. 36, 1510 (1966); and C. H. Thomas et al., IEEE J. Quantum Electron, QE-2, 617 (1966).
A summary of techniques for reducing amplitude fluctuations of laser output appears in W. Koechner, Solid-State Laser Engineering (Springer-Verlag, New York, 1976), pp. 91-97, 229-232.
Besides the above-described techniques, which have been described in the context of laser technology, a great deal of research has been directed to the development of materials that darken on being irradiated with electromagnetic radiation of various wavelengths. Generally, these "photochromic" materials involve chemical reactions that take place on a longer time scale than is necessary for reducing high-frequency laser amplitude fluctuations. Some photochromic materials respond very rapidly to actinic irradiation (see, e.g., S. A. Krysanov et al., Chem. Phys. Lett. 91, 77 (1982)); however, irradiation at another wavelength is required to bleach the darkened material.
For some laser applications it is desirable to have very short pulses. To accomplish this, a "saturable absorber" may be inserted into a pulsed laser beam path. A saturable absorber absorbs the initial part of an incident pulse, but passes the later part, thereby providing a transmitted pulse that is shorter than the incident pulse. (See, e.g., P. G. Kryukov and V. S. Letokhov, IEEE J. Quantum Electron. QE-8, 766 (1972)).
A particular example of a laser that achieves very short pulses through the use of a saturable absorber is a passively mode-locked laser. This type of laser has been known for decades and is described in Koechner, op. cit., pp. 461-474.